JP Laid-open No. 6-281799 discloses that a solid target provided in the shape of a winding tape is irradiated with a laser beam so as to generate X-rays.
JP Laid-open No. 61-153935 discloses that a droplet of liquid metal is irradiated with a laser beam so as to generate X-rays.
JP Laid-open No. 2-100297 discloses that a bullet shaped target having a diameter smaller than a spot diameter of a laser beam is irradiated with a laser beam so as to produce X-rays.
JP Laid-open No. 57-41167 discloses that solidified rare gas or water particles are irradiated with a laser beam so as to produce X-rays.
OSA, Trends in Optics and Photonics, vol. 4, EXTREME ULTRAVIOLET LITHOGRAPHY, 1996, pp. 66- discloses that a pressurized gas is injected into a vacuum chamber, and then the injected gas is irradiated with a laser beam so as to produce X-rays.
When a target is irradiated with a laser beam, atoms and molecules in the target are subjected to optical breakdown so as to be ionized, thereby producing a laser plasma. Then, an X-ray is generated from the laser plasma which has been produced. Depending on the species of elements in the target and the status thereof, the intensity of the laser beam required for the optical breakdown will change. The minimum limit value of laser beam intensity which causes optical breakdown is referred to as the breakdown threshold.
Optical breakdown thresholds become higher in the order of (1) gas, (2) liquid and (3) solid. In other words, the laser beam intensity required for producing a plasma being the same number of atoms can be lowered by using solid and/or liquid targets, rather than by using a gas target. Therefore, in the case where liquid and solid targets are used, the X-ray conversion efficiency (expressed in energy of X-rays generated relative to energy of irradiated laser beam) becomes higher than in the case where a gas target is used.
However, when a solid target in a bulk or a liquid target having a diameter larger than the spot diameter of a laser beam is used, the heat generated in the region of the target irradiated by the laser beam is conducted to its peripheral region, thereby melting the peripheral region thereof. Then, the melted portion is caused to scatter in response to an expansion pressure associated with the production of the laser plasma. This scattered portion, which is called debris, assumes various forms, such as an ionized state, cluster state and a particle of several tens Am in size. This debris attaches to and damages optical elements in the vacuum chamber.
When solid and liquid targets are provided in the form of a particle having a diameter which is smaller than the spot diameter of the laser beam, the amount of debris will be reduced due to absence of particles in the periphery of the irradiated portion, with the same X-ray conversion efficiency being ensured as by using the solid and liquid targets. However, it is very difficult to supply a target in the shape of particles in synchronism with irradiation of a laser beam, and thus, it is difficult to produce X-rays stably.
A target of particles prepared by freezing a chemically stable gas, such as one of the rare gases, will not produce debris when melted, since it turns into a chemically stable gas. However, it is difficult to supply such targets, thereby preventing stable production of X-rays. Further, since the frequencies of its specified X-ray are limited, and if there exists a difference from a desired wavelength, the X-ray conversion efficiency of the target becomes substantially lower than that of a metal target.
On the other hand, when using a gaseous target, since the target has a smaller heat conduction to its peripheral portion as compared to solid and liquid targets, thus not likely causing melting of the peripheral portion, a continuous supply of such a gaseous target can be ensured with minimum debris, thereby being capable of stably producing X-rays. However, since its breakdown threshold is higher, and its atomicity density is lower, its X-ray conversion efficiency is lower than those of the solid and liquid targets.